Link scheduling queuing
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Link Scheduling & Queuing. COS 461: Computer Networks http://www.cs.princeton.edu/courses/archive/spr14/cos461/. Outline. Link scheduling (not covered on Monday) Queuing practice questions Miscellanea. First-In First-Out Scheduling. First-in first-out scheduling Simple, but restrictive

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Link Scheduling & Queuing

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Link scheduling queuing

Link Scheduling & Queuing

COS 461: Computer Networks

http://www.cs.princeton.edu/courses/archive/spr14/cos461/


Outline

Outline

  • Link scheduling (not covered on Monday)

  • Queuing practice questions

  • Miscellanea


First in first out scheduling

First-In First-Out Scheduling

  • First-in first-out scheduling

    • Simple, but restrictive

  • Example: two kinds of traffic

    • Voice over IP needs low delay

    • E-mail is not that sensitive about delay

  • Voice traffic waits behind e-mail

3


Strict priority

Strict Priority

  • Multiple levels of priority

    • Always transmit high-priority traffic when present

  • Isolation for the high-priority traffic

    • Almost like it has a dedicated link

      • Except for (small) delay for transmission

  • Possible starvation

High

Low

4


Weighted fair queuing

Weighted Fair Queuing

  • Each queue gets a fraction of the link bandwidth

  • Rotate across queues on a small time scale

  • What if a queue is empty during its time slice?

    • Move on to next queue if work conserving

50% red, 25% blue, 25% green

5


Weighted fair queuing1

Weighted Fair Queuing

  • If non-work conserving

    • Flows get at most their allocated weight

  • If work-conserving

    • Send extra traffic from one queue if others are idle

    • Bytes, not packets

    • Higher or lower utilization than non-work conserving?

  • Results in max-min fairness

    • Maximize the minimum rate of each flow

6


Implementation trade offs

Implementation Trade-Offs

  • FIFO

    • One queue, trivial scheduler

  • Strict priority

    • One queue per priority level, simple scheduler

  • Weighted fair scheduling

    • One queue per class, and more complex scheduler

7


Link scheduling queuing

For the following questions, assume that these are always coupled with a FIFO scheduling policy.

The full queue problem occurs if routers’ queues are often full.

The lockout problem refers to a situation where a small number of flows monopolize the available queue space on a router.

  • Drop-tail solves the full queue problem T/F


Link scheduling queuing

For the following questions, assume that these are always coupled with a FIFO scheduling policy.

The full queue problem occurs if routers’ queues are often full.

The lockout problem refers to a situation where a small number of flows monopolize the available queue space on a router.

  • Drop-tail solves the full queue problem T/F


Link scheduling queuing

For the following questions, assume that these are always coupled with a FIFO scheduling policy.

The full queue problem occurs if routers’ queues are often full.

The lockout problem refers to a situation where a small number of flows monopolize the available queue space on a router.

  • Drop-tail solves the full queue problem T/F

  • Random Early Detection (RED) solves the full queue problem T/F


Link scheduling queuing

For the following questions, assume that these are always coupled with a FIFO scheduling policy.

The full queue problem occurs if routers’ queues are often full.

The lockout problem refers to a situation where a small number of flows monopolize the available queue space on a router.

  • Drop-tail solves the full queue problem T/F

  • Random Early Detection (RED) solves the full queue problem T/F


Link scheduling queuing

For the following questions, assume that these are always coupled with a FIFO scheduling policy.

The full queue problem occurs if routers’ queues are often full.

The lockout problem refers to a situation where a small number of flows monopolize the available queue space on a router.

  • Drop-tail solves the full queue problem T/F

  • Random Early Detection (RED) solves the full queue problem T/F

  • RED solves the lockout problem for TCP flows T/F


Link scheduling queuing

For the following questions, assume that these are always coupled with a FIFO scheduling policy.

The full queue problem occurs if routers’ queues are often full.

The lockout problem refers to a situation where a small number of flows monopolize the available queue space on a router.

  • Drop-tail solves the full queue problem T/F

  • Random Early Detection (RED) solves the full queue problem T/F

  • RED solves the lockout problem for TCP flows T/F


Link scheduling queuing

1 Drop-tail has fewer burst losses than RED

T/F


Link scheduling queuing

1 Drop-tail has fewer burst losses than RED

T/F


Link scheduling queuing

1 Drop-tail has fewer burst losses than RED

T/F

2 Both drop-tail and RED can be used with Explicit Congestion Notification (ECN), so the router can signal congestion to the sender without dropping a packet

T/F


Link scheduling queuing

1 Drop-tail has fewer burst losses than RED

T/F

2 Both drop-tail and RED can be used with Explicit Congestion Notification (ECN), so the router can signal congestion to the sender without dropping a packet

T/F


Link scheduling queuing

1 Drop-tail has fewer burst losses than RED

T/F

2 Both drop-tail and RED can be used with Explicit Congestion Notification (ECN), so the router can signal congestion to the sender without dropping a packet

T/F

3 RED drops every incoming packet with some probability P > 0

T/F


Link scheduling queuing

1 Drop-tail has fewer burst losses than RED

T/F

2 Both drop-tail and RED can be used with Explicit Congestion Notification (ECN), so the router can signal congestion to the sender without dropping a packet

T/F

3 RED drops every incoming packet with some probability P > 0

T/F


Tcp windows

TCP Windows

  • Receive window

    • flow control

  • Congestion window

    • Additive increase / multiplicative decrease

    • Triple duplicate ACKs

    • Slow-start, fast retransmit, fast recovery, etc.


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